Optical lithography involves the creation of relief image patterns through the projection of radiation within or near the ultraviolet (UV) visible portion of the electromagnetic spectrum. Techniques of optical microlithography have been used for decades in the making of microcircuit patterns for semiconductor devices. Early techniques of contact or proximity photolithography were refined to allow circuit resolution on the order of 3 to 5 μm. More modern projection techniques minimize some of the problems encountered with proximity lithography and have lead to the development of tools that currently allow resolution below 0.15 μm.
Semiconductor device features are generally on the order of the wavelength of the UV radiation used to pattern them. Currently, exposure wavelengths are on the order of 150 to 450 nm and more specifically 157 nm, 293 nm, 248 nm, 365 nm, and 436 nm. The most challenging lithographic features are those which fall near or below sizes corresponding to 0.35 λ/NA, where λ is the exposing wavelength and NA is the objective lens numerical aperture of the exposure tool. As an example, for a 193 nm wavelength exposure system incorporating a 0.75 NA objective lens, the imaging of features at or below 90 nanometers is considered state of the art.
The feature size is proportional to the illumination wavelength and inversely proportional to the numerical aperture (NA) of the lithography system. The absolute limitation to the smallest feature that can be imaged in any optical system corresponds to 0.25 λ/NA. Furthermore, the depth of focus (DOF) for such an exposure tool can be defined as +/−k2 λ/NA2 where k2 is a process factor that generally takes on a value near 0.5
Optical lithography has been driven toward sub-100 nm dimensions using techniques such as high NA, phase-shift masking, modified illumination, optical proximity correction, and pupil filtering are being employed. Fabrication challenges have been aggressively pursued so that 70 nm device geometry may be possible using wavelengths as large as 193 nm. Lithography at 157 nm is positioned for 50-70 nm technology, extending optical methods yet further along the semiconductor technology roadmap. Though the shorter wavelength of 157 nm is beneficial, additional resolution enhancement is needed to ensure that this technology is viable or is multi-generational. The problems with such a short wavelength have introduced associated risks.
If numerical aperture (NA) values above 1.0 were possible by imaging into a media with a refractive index greater than 1.0, sub-quarter wavelength lithography could be obtained. Such large numerical apertures have been demonstrated using immersion methods for microscopy and more recently for lithographic applications. Exploiting this potential at 157 nm has been shown by imaging into perfluoropolyethers, but these fluids are generally too absorbing to allow for application to manufacturing.
The choice of an immersion fluid is based primarily on its transparency. As wavelengths are increased from the vacuum ultraviolet (VUV), liquids with more attractive properties have sufficient transmission for use as immersion imaging fluids. The prime example is water, with absorption below 0.50 cm−1 at 185 nm and below 0.05 cm−1 at 193 nm. The refractive index of water at 193 nm is 1.44, which effectively decreases wavelength to 134 nm. The resolution is proportionally increased by the refractive index value. This represents a 43% potential improvement in resolution, which is twice that achieved with the wavelength transitions from 248 nm to 193 nm, from 193 nm to 157 nm, or from 157 nm to 126 nm. DOF for immersion imaging is calculated based on the effective reduction in wavelength, or λ(nNA2). This is significant as the usable focal depth scales linearly with the media index rather than quadradically with the media NA.
Immersion lithography has been explored in the past, as disclosed in U.S. Pat. Nos. 4,480,910 and 4,509,852 which are both herein incorporated by reference in their entirety, with little success for several reasons including, UV resists used for wavelengths above 300 nm release a significant volume of dissociated nitrogen through photochemical reaction upon exposure. Additionally, the relatively high index fluids at UV wavelengths above 300 nm tend to react with photoresist materials. Further, standard immersion fluids used in microscopy are not transparent below 300 nm. Also, alternative microlithography solutions existed. Furthermore, the refractive index of water at UV wavelengths above 300 nm is not sufficiently high (˜1.30) to warrant its use so that the above concerns could be alleviated.
The current situation for deep ultraviolet/vacuum ultraviolet (DUV)/VUV lithography is quite different. Optical lithography is approaching the limits of wavelength, resolution, and conventional numerical aperture. Exploration into imaging methods that were previously considered impractical have become reasonable. The use of water as an immersion fluid is now an attractive choice for several reasons that are contrary to the problems in the past. For example, 193 nm and 248 nm resist platforms release low volumes of gas during exposure. Additionally, the reaction of water with 193 nm and 248 nm photoresists is minimal and can be reduced to immeasurable levels through modification of pH. Further, water is transparent to below 0.05 cm−1 at 193 nm and water is an existing component of wafer processing, limiting the critical concerns of wetting, cleaning and drying. Also, few alternative optical choices now exist. Although water can be utilized as an immersion fluid at wavelengths between 180 and 300 nm, it is limited to refractive index at wavelengths between 1.40 and 1.45. Larger immersion fluid refractive index values would be preferred.
Predictions of the International Technology Roadmap for Semiconductors (ITRS) for future lithography requirements are in a table shown in FIG. 1. The recent interest in immersion lithography is allowing for significant progress along this roadmap. This is made possible by enhancing the effective numerical aperture of optical lithography systems. If numerical aperture values above 1.0 are made possible using immersion imaging, resolution to 32 nm could be obtained. By extending the usability of UV wavelengths, the materials and availability issues associated with shorter vacuum-UV (VUV) wavelengths are relaxed. Immersion lithography is being established as the most effective way to push UV and VUV lithography into additional device generations. The critical research involved can be very economical and the technology represents relatively small changes in the optical tooling requirements.